Add backend unit tests for rej_uniform and poly compress/decompress

Add native-vs-C consistency tests for previously untested backends:
- mlk_rej_uniform_native: compare against mlk_rej_uniform_c
- mlk_poly_compress_d{4,5,10,11}_native: compare against C reference
- mlk_poly_decompress_d{4,5,10,11}_native: compare against C reference

These tests call the assembly backends directly with heap-allocated
buffers, enabling valgrind to detect buffer overflows. In particular,
the rej_uniform test would have caught the 4-byte overread in the
AVX2 rejection sampling fixed in commit f10b801.

Previous ad-hoc tests for detecting overflow in (de)compression
routines are now subsumed by the unit tests, and removed.

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>

Author: hanno-becker